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Vitamin A

Other name(s):

General description

Vitamin A, or retinol, was the first substance isolated in the group called vitamins. It is one of the four fat-soluble vitamins and is found in animal products. Fat-soluble vitamins are stored in the body and can accumulate to toxic levels when intake is excessive.

Precursors of vitamin A exist in a variety of plants and are called carotenoids. Carotenoids are fat soluble but non-toxic, even in large quantities. The best known carotenoid is beta-carotene. Both retinoids and carotenoids are good antioxidants.

Vitamin A plays an important part in the reproductive process. It is involved in the development of sperm (spermatogenesis) and also in the development of the fetus. However, excessive doses of vitamin A and synthetic retinols may lead to fetal malformation and congenital abnormalities. Vitamin A appears to guide the developing structures in the fetus. It also helps in the development of the placenta during pregnancy.

In addition to reproductive health, vitamin A is an important factor in growth throughout life. Vitamin A is involved in the development and maintenance of epithelial tissues (mucus membranes, the lining of the gastrointestinal tract, lungs, bladder, urinary tract, vagina, cornea, and skin). Vitamin A also helps the growth of bones and teeth.

Vitamin A also prevents drying of the skin, which in turn helps protect the body from infectious diseases. Vitamin A is also involved in maintaining the immune system (T-lymphocytes and phagocytosis).

Finally, vitamin A is essential for night vision. Retinol (a vitamin A metabolite) combines with opsin (a pigment in the retina of the eye) to form rhodopsin, a chemical involved in night vision.

Medically valid uses

Vitamin A plays an important role in the reproductive process, growth and development, maintenance of healthy eyes and skin, and as an antioxidant.

There is mounting evidence that vitamin A supplements may reduce the risk for certain types of cancer. The ability of vitamin A to influence cell development, coupled with its ability to increase the activity of immune-system cells, could make it a valuable tool in the fight against cancer—especially skin, lung, bladder, and breast cancer.

Unsubstantiated claims

Please note that this section reports on claims that have NOT yet been substantiated through scientific studies.

Vitamin A is claimed to prevent some types of cancer and acne, and to help treat psoriasis. It is also claimed to be effective for treating dry or wrinkled skin, and protecting against the effects of pollution, healing sunburns, and preventing respiratory infections. Vitamin A has also been used to treat kidney stones, inflammatory bowel disease, and deafness.

Vitamin A has been called the anti-infection vitamin because of its role in helping the body combat bacterial, parasitic, and viral infections.

Again, the above claims are NOT supported in current research findings.

Recommended intake

As indicated below, vitamin A is measured in International Units (IU). The RDA is the Recommended Dietary Allowance.

Group

RDA (IU)

Infants (0 to 1 year)

1,875 IU

Children (1 to 3 years)

2,000 IU

Children (4 to 6 years)

2,500 IU

Children (7 to 10 years)

3,500 IU

Men (11+ years)

5,000 IU

Women (11+ years)

4,000 IU

Pregnancy

4,000 IU

Breastfeeding (first 6 months)

6,500 IU

Breastfeeding (second 6 months)

6,000 IU

Smaller individuals need relatively less vitamin A than larger people do. Similarly, women generally need less than men, except during pregnancy and while breastfeeding.

Food source

Nutrient content per 100 grams

Cod liver oil

85,000 IU

Beef liver

20,000 IU

Carrots

11,000 IU

Sweet potatoes

8,800 IU

Parsley

8,500 IU

Spinach

8,100 IU

Cantaloupes

3,400 IU

Apricots

2,700 IU

Vitamin A is stable at room temperature and therefore does not need to be refrigerated. Cooking, freezing, or canning vegetables and fruits does not destroy an appreciable amount of vitamin A. However, vitamin A tablets and capsules should not be frozen.

Vitamin A is also stable in light.

A poor diet with inadequate intake of vitamin A or a diet containing large amounts of snack foods containing the fat substitute Olestra can increase the need for vitamin A.

Malabsorption syndromes characterized by steatorrhea (excess fat in the stool) may deplete all four fat-soluble vitamins: A, D, E, and K. Common malabsorption syndromes include lactose intolerance, tropical and non-tropical sprue, celiac disease, cystic fibrosis, ulcerative colitis, Crohn's disease, and steatorrhea caused by pancreatectomy (removal of all or part of the pancreas).

Other reasons for increased need for vitamin A include prolonged fever, hyperthermia, infection, chronic use of mineral oil, diabetes, or hyperthyroidism.

An early sign of vitamin A deficiency is night blindness. This condition may begin with decreased ability to see at night or in the dark. The time required for eyes to adjust to the dark increases until the person loses the ability to see at night.

As vitamin A deficiency continues, xerophthalmia (conjunctival dryness) may develop, beginning with a dry feeling in the eyes. The eyelid linings also become dry (xerosis conjunctivae) and rough. As the condition continues, the cornea dries out (xerosis cornea) and becomes wrinkled and cloudy. Scarring then occurs with irreversible changes leading to blindness.

Skin changes are another sign of too little vitamin A. The skin becomes dry and rough (follicular hyperkeratosis). This is very noticeable over the shoulders, buttocks, and the extensor surfaces of the arms and legs. Little bumps may appear around the base of each hair, causing a sandpaper-like texture to the skin.

Mucous membranes may also change and affect the lining of the urinary tract, causing burning with urination and the appearance of blood. The lining of the vagina may also become dry and inflamed.

Retinol is used to treat vitamin A deficiency. In many undeveloped countries, vitamin A deficiency is common. Because vitamin A can be stored in the body, large doses can be given to children (and some adults) only two or three times a year to prevent xerophthalmia, a condition that leads to blindness. Vitamin A deficiency is rare in the United States. When it does occur it is usually due to malabsorption caused by other diseases.

Side effects, toxicity, and interactions

Vitamin A in the form of beta-carotene is considered safe and does not appear to be toxic in large doses. However, high doses over a long period of time can lead to a condition known as carotenemia, in which the skin becomes yellowish orange.

Vitamin A overdose in the form of retinoids (from animal sources) can be toxic. The conditions of overdose, or hypervitaminosis, are divided into two groups: acute and chronic, which are further separated into infant and adult.

Children are more sensitive than adults to overdoses of vitamin A. The symptoms of an acute overdose in an infant or child are nausea and vomiting, drowsiness, or excessive sleepiness. Other signs are a bulging soft spot (fontanel) on the top of the infant's head and a condition called pseudotumor cerebri. This condition produces increased pressure around the brain, bulging of the optic disc in the back of the eye, paralysis, or change in function of some of the cranial nerves. (This condition is seen after the soft spot has closed over and the sutures fused.)

The symptoms of a chronic overdose in an infant or child are loss of appetite, itching (pruritus), decreased weight gain, irritability, and hair loss (alopecia). Additional signs are tenderness in the bones (especially long bones) and noticeable swelling. Skin changes such as dryness, roughness, and cracks in the corners of the mouth may be present.

In adults, symptoms of an acute overdose may include headache, nausea, vomiting, irritability, seizures, and double vision (diplopia).

The signs of a chronic overdose in adults include skin changes such as dryness, roughness, and fissuring of the lips and corners of the mouth. Additional signs may be hair loss (alopecia), loss of appetite and increased intracranial pressure (pressure around the brain), which may cause symptoms similar to those of a brain tumor.

Do not take vitamin A supplements if you are hypersensitive to vitamin A.

Megadoses of vitamin A should be avoided during pregnancy as they may cause malformations in the developing fetus.

Mineral oil, cholestyramine, and foods containing Olestra may interfere with the absorption of vitamin A. Orlistat, a drug for weight loss, has been shown to decrease absorption of beta-carotene and vitamin E. Whether Orlistat has the same effect on vitamin A is still unknown.

Patients taking Accutane (isotretinoin), Soriatane (acitretin), or Tegison (etretinate) should not use vitamin A supplements.

Retin A and Differin Gel are topical ointments, but since they are in the retinoid family, supplements of vitamin A should be used with caution when using these preparations.

Tetracycline and megadoses of vitamin A have caused benign intracranial hypertension, so avoid taking vitamin A in conjunction with this drug.

Oral contraceptives increase the levels of vitamin A in the bloodstream, therefore vitamin A supplements may be unnecessary.

Additional information

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